The present invention relates to planographic printing plates for use in dry planography in which the conventional use of dampening water is obviated. The invention also relates to a method for preparing such printing plates.
Planographic printing, unlike letter-press or gravure printing, is carried out by use of printing plates of planographic type, i.e., not containing raised or depressed areas as the ink-holding areas but containing image and non-image areas provided substantially on the same plane. The ordinary process of planographic printing which is based on the principle that water and oil-based printing ink are immiscible to each other, comprises the following steps.
1. Non-image areas are rendered hydrophilic by a chemical or mechanical treatment, while image areas are made ink-receptive by application of an oleophilic resin by means of transferring or photographic techniques.
2. The printing plate is dampened with water, thus the hydrophilic non-image areas becoming wet while the oleophilic image areas repelling the dampening water.
3. Then, the plate is inked, leaving a film of ink on the image areas only, so that this film of ink can be transferred imagewise to another surface brought into contact therewith, such as paper.
The disadvantages experienced in the practice of the conventional planographic printing process are, for example, firstly that the dampening water applied to the plate may flow back to the inking rollers on the press during printing operations, whereby the ink becomes emulsified causing soiling to the paper sheet, secondly that the dampening water transferred to the printing material may be a cause of its dimensional changes, being of a serious problem especially in multiple color printing where the accurate registration of different colors is essential, and thirdly that the stability or uniformity in color tone is sometimes unsatisfactory due to difficulties encountered in securing delicate balance between the amounts of the dampening water and of the ink to be used.
In order to overcome the above-described disadvantages, various attempts were made for the develment of planographic printing processes in the absence of the dampening water, but no satisfactory results were obtained. According to one example of the attempts, a planographic printing plate was prepared by forming a layer of a composition with a photosensitive diazo compound on the base of, say, aluminum and overlying a layer of dimethylpolysiloxane gum. The plate was overlaid with a nagative transparency and exposed to light to insolubilize the photosensitive diazo layer in the exposed areas, followed by development by washing away the diazo composition from the unexposed areas and finished by removing the layer of the dimethylpolysiloxane gum off the unexposed areas, leaving the layer of the dimethylpolysiloxane gum imagewise on the exposed areas corresponding to the non-image areas owing to the ink-repellency of the siloxane gum (see British patent specification No. 1,146,618).
Another example of the past attempts was such that a planographic printing plate was prepared by providing on a substrate of, say, aluminum successive layers of a photosensitive diazo composition, an adhesive and a silicone rubber, and overlaying the thus prepared plate was a positive transparency, followed by exposure to light to decompose the photosensitive diazo compound on the exposed areas and stripping the silicone rubber layer imagewise on the exposed areas (see U.S. Pat. No. 3,511,178).
The prior art planographic printing plates referred to above were found defective in some important points. For example, the intervention of the silicone rubber layer between the photosensitive diazo layer and the positive or negative transparency made it difficult to perform the imagewise exposure of the photosensitive layer with high image fidelity. As another example, the image patterns formed by the silicone rubber layer left behind did not always exhibit satisfactory sharpness at their edges, since the stripping of the silicone rubber layer was carried out relying on the change of the solubility of the layer in a solvent. As a further example, the processing of the prior art planographic printing plates was complicated because of the necessity of providing a plurality of layers successively onto a substrate before subjecting to exposure to light and developing.
The inventors of the present invention then performed extensive investigations with the purpose to produce planographic printing plates free from the above described problems. As a result, such planographic printing plates were proposed that had a single layer of a photopolymerizable composition composed of a photopolymerizable organopolysiloxane, a photosensitizer and a solvent, on their aluminum or other bases or substrates. The printing plate was mounted in contact with a transparency of positive patterns, and exposed to light to photopolymerize the organopolysiloxane on the exposed areas thereby to convert to an insoluble, cured film having excellent strength, ink-repellency, heat-stability, chemicals resistance and anti-corrosiveness, followed by development with a solvent to wash the organopolysiloxane layer off the unexposed areas, leaving the cured organopolysiloxane layer imagewise on the exposed areas (see Japanese Pat. No. 33,910/1973).
However, the above planographic printing plates were then found defective in that the photopolymerizable organopolysiloxane was not sufficiently compatible with the photosensitizer, resulting in the precipitation of the latter ingredient from the former within the layer formed and consequently in resulting in the lowering of the photosensitivity of the composition.
In order to provide a remedy for the abovedescribed defects, the same inventors then proposed renewed planographic printing plates in which the photosensitive layer was composed of a photopolymerizable organopolysiloxane and a silicon-containing benzophenone derivative as a photosensitizer. These new printing plates were suitable for a similar processing to the old one and advantaged in that the compatibility inherent between the silicon-containing benzophenone derivative and the photopolymerizable organopolysiloxane served to reduce the precipitation of the photosensitizer from the organopolysiloxane, and the lowering of the photosensitivity or the occurrence of fogging due to heat could be prevented even on prolonged storage before exposure to light. The printing plates were found quite useful in planography in the absence of dampening water, with the resulting prints being sharp and precise.
Despite the improvement achieved by the siliconcontaining benzophenone derivative as a photosensitizer was remarkable, there was still the questions unsolved as to the compatibility of the photosensitizer with the photopolymerizable organopolysiloxane which was not sufficient and the precipitation of the former from the latter which worked to lower the photosensitivity.
Under the circumstances, the inventors of the present invention proposed a further improvement such that the possibility of the precipitation of the photosensitizer from the organopolysiloxane should be completely eliminated by the use of a self-sensitizing photopolymerizable organopolysiloxane having both photopolymerizable and photosensitizing groups in a molecule. However, such self-sensitizing photopolymerizable organopolysiloxane have encountered difficulties in the manner of synthesizing them wherein it is required to make the organopolysiloxane effectively react with the groups having the photopolymerizing and photosensitizing properties.